Cellulose esters are used in various applications such as filter tow, pharmaceutical coatings, ink compositions, molded articles and industrial coatings. Thus, methods for their economical preparation are clearly desirable. Coating compositions employing ordinary cellulose esters frequently exhibit relatively low solids contents, i.e., the compositions require excessive amounts of solvents which present difficulties with regard to drying time, solvent evaporation and other environmental concerns. Although, solvent borne cellulose esters may provide desirable properties to coatings, recent trends toward lowering the volatile organic compound (VOC) of coating formulations have reduced the use of solvents, and thereby solvent-borne cellulose esters. Further, ordinary cellulose esters exhibit limited compatibility with some classes of polymeric materials.
One way to improve the compatibility of cellulose esters is to add carboxylic acid groups to the cellulose ester. A process for carboxylating cellulose ester has been described by I. D. Sand in "The Properties and Applications of Ozonolyzed Cellulose Acetate Butyrate," Polymer Material Science Engineering, 1987, pp. 57-63. In this process the hydroxyl groups of cellulose acetate butyrate are oxidized in the presence of ozone to produce an oxidized or carboxylated cellulose acetate butyrate. According to Sand, carboxylated cellulose acetate butyrates have a lower molecular weight than conventional non-carboxylated cellulose acetate butyrates. As a result, the carboxylated cellulose acetate butyrates are soluble in a broader range of solvents and are compatible with a wider range of resins than the conventional cellulose acetate butyrates. Unfortunately, however, in addition to the carboxyl group, the ozone treatment described by Sand also generates peroxide, aldehyde and ketone functional groups in the cellulose ester.
Similarly, European Patent Application No. 138,703 describes a process for preparing carboxylated cellulose esters by reacting a cellulose ester with ozone. However, like the process described by Sand, in the process described by this European Patent Application, peroxide, aldehyde, and ketone functional groups are generated in the cellulose esters in addition to the carboxyl group.
Thus, there remains a need to develop cellulose esters which can be formulated into high solid coating compositions satisfying current VOC requirements, while maintaining solvent solubility and coating resin compatibility. The carboxylated cellulose esters of the invention answer this need.